Technical Field
The subject invention relates to a chromate/silicate surface treatment for providing an aluminum heat exchanger with corrosive-resistent hydrophilic coatings.
Heat exchangers, such as evaporators and radiators, are often surface treated with a chromate solution to provide a corrosive-resistant chrome coating thereon. Moreover, heat exchangers may be additionally treated with a silicate solution for providing a hydrophilic coating. The hydrophilic coating breaks the surface tension of water; thus, water tends to run off and not collect or bead-up on the surface of the heat exchanger.
Such a surface treatment is available from Circle-Prosco Inc., of Bloomington, Ind. The Circle-Prosco treatment process includes providing a plurality of liquid baths arranged serially in which an aluminum heat exchanger is sequentially immersed. More specifically, the Circle-Prosco process involves providing a cleaning bath including an aqueous acid cleaning solution. The acid cleaning solution includes 40-60 weight percent of nitric acid and 5-1 percent of sodium fluoride, (commercially available from Circle-Prosco as Acid Cleaner 8D). The acid cleaning solution is diluted with water so that the overall concentration of the acid cleaning solution in the cleaning is between 2.0-5.0 weight percent. The temperature of the cleaning solution is maintained between 120.degree.-135.degree. F.
At least one and preferable several rinse water baths are provided sequentially following the cleaning bath for rinsing the cleaning solution from the cleansed heat exchanger. Following the rinse water baths, a chromate bath is provided including an aqueous chromate solution. The chromate solution is an aqueous solution comprising 10 weight percent of chromium trioxide, 25 weight percent of nitric acid and 10 weight percent of hydrofluoric acid, (commercially available from Circle-Prosco as Alcoat 300 BD). The chromate solution is diluted with water so that the overall concentration of the chromate solution in the chromate bath is between 1.8-3.3 weight percent. The chromate bath further comprises a chromate activator including an aqueous solution of less than 25 weight percent of molybdic acid and disodium salt (commercially available from Circle-Prosco as Alcoat 300 AD). The chromate activator solution is diluted so that its overall concentration within the chromate bath is between 1.2-1.5 weight percent. The temperature of the chromate bath is maintained between 120.degree.-135.degree. F.
At least one and preferable several rinse water baths are provided sequentially following the chromate bath for rinsing the chromate solution from the chromed heat exchanger. Finally, a silicate bath is provided including a silicate liquid. The silicate liquid comprises 90-100 weight percent of silicate of soda and 0-10 weight percent of potassium hydroxide (commercially available from Circle-Prosco as Final Rinse 8D). The silicate liquid is diluted with water so that its overall concentration within the silicate bath is between 2.3-5.0 weight percent. The silicate bath is maintained between 155.degree.-165.degree. F.
The Circle-Prosco process includes immersing an aluminum heat exchanger within the cleaning bath to remove any oxidation from the heat exchanger. Subsequently, the heat exchanger is removed from the cleaning bath and immersed in the rinse bath(s) to remove any residual cleaning solution therefrom. Once rinsed, the heat exchanger is immersed within the chromate bath where it is provided with a chrome coating. Subsequently, the heat exchanger is removed from the chromate bath and immersed in the rinse bath(s) to remove any residual chromate solution therefrom. Finally, the heat exchanger is immersed within the silicate bath wherein the heat exchanger is provided with a hydrophilic coating. The heat exchanger is removed from the silicate bath and is then moved to a dry oven where the coatings on the heat exchanger are fully dried.
A common problem with heat exchanges, including those treated with the Circle-Prosco process or similar processes, is the emission of a musty odor.
Microscopic organisms are often present within heat exchangers due to the presence of moisture, and as a result, cause a musty odor to be emitted from the heat exchanger. As a consequence, much attention has been directed toward treatment of heat exchangers to prevent fungal growth. For example, U.S. Pat. Nos. 4,672,816 to Takahashi and 3,762,178 to Yamada disclose air conditioning systems wherein an air blower is pre-cooled prior to blowing cooled air into the passenger compartment in order to reduce musty odor problems. Further examples include the use of fungicides to prevent fungal growth.
Regardless of the use of fungicides or other remedial measures, heat exchangers treated with chromate/silicate processes as described above, continue to emit a musty smell.